This study seeks to explore the possible relationship between blood hormone levels and the changes in body composition which take place during adolescence in man. Estimates of lean body mass and body fat content will be made by potassium-40 counting in normal children aged 7- 18 years, and blood levels of testosterone, estrogens, LH and FSH by radioimmunoassay. In addition assessments will be made of height and weight, bone age, and the development of secondary sex characteristics. The hypotheses to be tested are: 1) that the increase in lean body mass which occurs in adolescence (most pronounced in boys) is related to the level of circulating androgens, and 2) that the increase in body fat (most pronounced in girls) is related to the level of estrogens. A subsidiary goal is the establishment of interrelationships among these various parameters, as well as their relationship to the clinical manifestations of adolescent growth and development.